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COMMS (Cummunication Subsystem) |
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The communications subsystem (COMMS) will perform two functions. Firstly, it will provide the telemetry downlink and telecommand uplink functionality. Besides that, it will provide a linear transponder service which can be used by amateur radio operators to communicate with each other. The satellite will have an uplink in the satellite segment of the UHF amateur radio frequency band (435-438 MHz) and a downlink in the satellite segment of the VHF amateur radio frequency band (145.8-146 MHz). Telemetry downlink will be done using 1200 bit/s BPSK modulation, using the amateur standard AX.25 protocol. Telemetry is encoded in UI frames. The COMMS consists of two systems, the Advanced Transceiver (ATRX) payload and a so-called Radio Amateur Platform. The Advanced Transceiver payload will be discussed in section VI. The Radio Amateur Platform functions as a backup option for the Advanced Transceiver. It is built using standard commercial-off-the-shelf (COTS) components, providing the same telemetry, telecommand and additionally the linear transponder functionality. The linear transponder will relay a 40 KHz wide passband from the UHF band to the VHF band. This provides a very flexible system, which can be used by amateur radio operators worldwide using CW, SSB and PSK31 modulation. The antenna subsystem consists of a two canted turnstile arrangements, one for each band. The antennas are fed in phase quadrature in order to achieve the desired pattern and polarization by means of a phasing circuit, which connects to both transceivers.
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